`skip checkup` should test an Android build without Skip

[dfabulich]

skip checkup initializes a fresh Skip app project and runs its build.

But, if we're running a checkup, then it's probably because we're unsure whether we can run any Android build.

skip checkup should generate a very basic Android project with no Skip in it at all, and verify that it builds without error. If that fails, there's no point in running a full-blown Skip project.

Failure in that step would narrow down the scope of the problem to just the user's Android SDK.